I had the same issue (specifically I could download books, but the current book I was reading would not sync the page to my computer nor my phone from my kindle -- the kindle would give an error as well), but the above steps actually did not work
The steps were similar to these, https://www.amazon.com/gp/help/customer/display.html?nodeId=201734170, which also did not work.
but while I as logged into my settings area in Amazon I looked for the book in question and it said there was an 'update' to my book. I updated and pushed the book to all three of my devices, no dice. So then I just removed the book from all devices, re-pushed to the three, and then it started working (sadly the page is now stuck at page 993 of 993 while only 56% complete with the book, but eh it syncs now)
so if you can download on your devices, the version of the book may be different and you may need to redownload the book (granted for me I needed to remove it from all devices then re-download, so mark the location in the book!)
hth